Rapidity means the quality of being very fast or swift. When a cheetah chases its prey across the savanna, the rapidity of its movements makes it almost a blur. When a skilled reader goes through a book with rapidity, they're moving quickly through the pages while still understanding what they read.

The word emphasizes remarkable speed that stands out. A student completing homework with rapidity is working unusually fast, finishing assignments in much less time than expected. Scientists might study the rapidity of a chemical reaction, noting how quickly substances combine or change.

You'll often see rapidity used when describing processes, changes, or movements that happen faster than expected or faster than normal. The rapidity with which a wildfire spreads can surprise firefighters. The rapidity of technological change over the past century transformed how humans live, from horse-drawn carriages to electric cars in just a few generations.

The related adjective is rapid, which means fast or quick: rapid growth, rapid progress, rapid response. When something happens with rapidity, it happens rapidly.